Electromyographic and mechanomyographic characteristics of neuromuscular block by magnesium sulphate in the pig

In spite of its well known propensity to cause accidental paralysis by a specific mechanism of action, Mg2+-induced neuromuscular block has not been examined systematically for its characteristics of muscle response to nerve stimulation. We examined in seven anaesthetized domestic pigs the mechanomyographic (MMG) and neurally evoked compound electromyographic (ncEMG, EMG) responses of the tibialis anterior muscle to stimulation of its motor nerve, at baseline and during three levels of neuromuscular block induced by infusion of MgSO4 (at approximately 25%, 50% and 75% depression of the 0.1 - Hz EMG). We observed that: at 0.1 Hz, the MMG tended to be more depressed than the EMG; the train-of-four (2 Hz) was essentially non-fading; the tetanic force (50 Hz) showed tetanic ascent instead of tetanic fade and reached its baseline control value at 5 s in spite of depression of the twitch; the EMG counterpart of the tetanus showed escalation of the train of ncEMG, so that the fourth ncEMG was much greater than the first; and the post-tetanic twitch was also relatively spared from Mg2+-induced neuromuscular block. Sparing of the tetanus and post-tetanic twitch resulted in large gains in the tetanus:twitch ratio and the post-:pre-tetanic twitch ratio, which increased at the 75% level of depression from 2.8 (SD 0.7) to 11.5 (4.0), and from 1.5 (0.3) to 4.6 (1.4) (P < 0.01), respectively. These characteristics of neuromuscular block by Mg2+ reflect its prejunctional mechanism of action by depression of transmitter release.